It is useful to be able to power up and power down devices based upon need, rather than have them powered all of the time. It is known to remotely power off a device that is functional and communicating, by sending a command that tells it to power off. The challenge comes in powering up a device that is powered off.
One can have a dedicated signal that connects directly to the power control, with associated proprietary control logic, to power on an associated device. Alternatively, the device may be in a reduced power mode with the device still able to communicate, limiting the power savings.